This invention relates to a method of preparing a silver halide photographic emulsion. More particularly, it relates to a method of preparing a ultra-slow speed silver halide photographic emulsion to be used for a lightroom light-sensitive material for making a graphic art film.
In recent years, in the field of graphic arts, there is an increase in colored and/or complex print products. Further, there has been developed a color scanner for which improved efficiency of a contact work has been particularly desired. Especially, the contact work in a light room has greatly contributed to the improvement in the operation efficiency and the proportion of the lightroom contact work has been increased year by year.
The lightroom contact work can be made practicable by both the improvement in mechanical factors such as contact printers and the improvement in light-sensitive materials.
Referring to light-sensitive materials, there has been developed a light-sensitive silver halide photographic material called a lightroom light-sensitive material, which can be handled in a light room and has a ultra-low sensitivity (1/10,000 to 1/100,000 of the sensitivity of a conventional film for darkroom contact work), and it has been attempted to provide materials of higher quality by imparting to the lightroom light-sensitive material the adaptability to lithographic development. However, none of the conventional materials have satisfied commercial needs.
On the other hand, in order to enhance the quality of the materials, it is considered useful to improve the method of preparing silver halide grains for the photographic emulsion. For example, it has been proposed to control pH conditions, pAg conditions, etc., and to improve a mixing method.
However, when these known methods of preparation of silver halide grains are applied in preparation of a photographic emulsion for the lightroom light-sensitive material, they are unsatisfactory from the viewpoints of gamma, fog and toe cut(herein meant by the contrast at the toe) in the photographic characteristic curve; dot quality; etc. To solve these problems, the present inventors have made studies about preparation method for silver halide grains which may have improved characteristics. As a result, they had an idea that such improved characteristics may be attained by providing a monodispersed emulsion.
By the way, the lightroom light-sensitive material herein mentioned is used at a light place (of illuminance of about 200 lux), and therefore the silver halide to be selected is set to have composition principally comprised of silver chloride in view of the sensitivity region. (This can be inferred from the disclosures in Japanese Unexamined Patent Publication Nos. 149030/1981 and 149031/1981).
In general, silver chloride is known to have a crystal growth rate which is faster than that of silver bromide and also to readily form cubic grains whose crystal habit have (100) face even when pH and pAg has varied. For this reason, a monodispersed emulsion can be readily obtained. On the other hand, it has been proposed in the above Japanese Unexamined Patent Publication Nos. 149030/1981 and 149031/1981 a lightroom light sensitive material employing a water-soluble rhodium salt as a desensitizer.
However, when the sensitivity of the silver halide grains has been set to a ultra-low sensitivity as used in the lightroom light-sensitive material, a conventional usual method for preparation of a monodispersed emulsion can not be applied as such to obtain silver halide grains being of high quality, in particular, high contrast and low fog. This is considered presumably due to the inhibition of formation and growth of the grains by the water-soluble rhodium salt used as a desensitizer.